Muscle and Bone: Partners for Life

Abstract
Originally, it was assumed that only a mechanical interaction existed between bone and muscle where the muscle pulled on the bone to allow movement. Now we know that each organ secretes factors, myokines and osteokines, that have effects on the opposing tissue. Bone and muscle are tightly intertwined throughout life, from development through aging. Genetic disease with mutations in muscle can have effects on bone and vice versa. With trauma, muscle accelerates bone healing.  With aging, frequently osteoporosis and sarcopenia occur simultaneously. Loaded bone produces factors that have positive effects on muscle generation and function such as prostaglandin E2, and Wnts, while resorbing bone produces factors such as Receptor activator of nuclear factor kappa-Β ligand, RANKL and Transforming Growth factor beta that have negative effects on muscle.  Conversely, static, resting muscle produces factors such as myostatin, which has not only negative autocrine effects but also has negative effects on bone.  Contracted or exercised muscle produces factors such as irisin and beta-aminoisobutyric acid, BAIBA, which have positive effects on bone but through very different receptors and signaling mechanisms. The combination of these biochemical signals with mechanical loading can synergize to increase bone or muscle mass. This synergy emphasizes the importance of exercise where not only is muscle contracted but in addition bone is loaded to maintain a healthy musculoskeletal system and whole body health.

Learning Objectives
1). Though both are mechanical organs, they also produce factors that have effects on the opposing organ.
2). Contraction of muscle and loading of bone induces anabolic factors, whereas static muscle and unloaded bone produce catabolic factors.
3). Muscle factors interact with suboptimal loading to induce bone formation.